In a business organization, information stored on your computer might be enormously momentous. Hence, regularly scheduled backups are obligatory, as sequential backups of the data are being done in order to provide speedy recovery during a disaster.

Once the first full backup of your Virtual Machines is over, the consequent incremental backup must transfer only the changed blocks of data rather than sending whole VM data again. This can be done with the help of CBT (Changed Block Tracking) which VMware has introduced since 2011.

Hyper-V lacks this Changed Block Tracking feature, where a third party backup software needs to assist in keep tracking of the changes in block level. Vembu BDR Suite provides File System Filter Driver to track the changes at block level of Hyper-V virtual machines effectively.

Ultimately, now Hyper-V has also introduced the much awaited feature that is named Resilient Change Tracking or (RCT) for tracking the changed blocks of the virtual machine disks. RCT is one of the new feature in Hyper-V for Microsoft Windows 2016, which eliminates the need for backup vendors to create their own change tracking filters for scanning the block changes.

How CBT tracking is done in Hyper-V

The VM needs to be scanned to track the changed disk blocks for incremental backup. One of the ways of achieving this is Changed Block Tracking (CBT) technology which drastically reduces the backup time. Changed block tracking creates a bitmap of all the blocks on a virtual hard drive. During backup, CBT driver checks the bitmap for all the changed blocks since the last backup and copies only those changed blocks.

CBT is implemented as a file system filter driver, where the driver is installed on every Microsoft Hyper-V host added to the backup infrastructure. The CBT driver keeps track of changed data blocks in virtual disks and stores these changes in log form, thereby reducing the number of backup storage and improving backup window times.

How changes tracking will be done in Windows Server 2016

At times of power failure or VM migration, the memory itself can be crashed and the whole bitmap file will be lost. In this scenario you will not have any knowledge of changes that took place thereafter. You’ll have to spend time and resources on doing a full rescan to backup afresh. RCT solves this problem by creating three bitmaps: one in memory and two on disk.
If you install Hyper-V server 2016 and create/migrate VMs, you see two files appear the first time you take a backup of a VM with configuration version 8.0 on Windows Server 2016 Hyper-V: a .MRT file and a .RCT file extension in the location of the virtual hard disks.

Resilient Change Tracking in Hyper-V

The Modified Region Table (.MRT) file maintains a record of changes on the Disk/Storage. This file has a coarser granularity, so that even at times of sudden power loss, the .MRT file records the changes occurred on the disk.

The Resilient Change Tracking (.RCT) file is used if the CBT data in the memory bitmap is not available at usual startup and shutdown operations of a VM. RCT file is also used for normal operating periods of backup, replication or migration. These .MRT and .RCT files are always attached to an associated VHD(x).

When the initial Backup is started, Microsoft Hyper-V takes a snapshot and Resilient Change Tracking identifiers creates a production reference point id for the VM. Similarly, for the incremental backups, it creates a reference point id for the changed blocks backup. Then, Microsoft Hyper-V compares the both reference ids of the full backup and the successive incremental backup. Once the comparison is done, it copies only the changed data blocks from the prior created checkpoints and records them in an incremental backup file.

Prior to RCT, Vembu BDR natively supports in securing the storage of the Hyper-V backups even at the time of migration using a filter driver. On putting forth the default RCT feature, Vembu increased its performance by using this RCT for Hyper-V backup and incremental changes tracking.

Beyond doubt, Microsoft Hyper-V Resilient Change tracking rapidly improves the backup and recovery speed of Hyper-V virtual machines in Window Server 2016 than before. The merit about this new feature is that the backup vendor could stop writing their own filter driver to implement changed block tracking.

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